Coarsening of intermetallic particles in rapidly solidified aluminium–transition-metal alloys

Abstract

The effect on microstructures of isothermal (200–625°C) and isochronal (0–1000 h) soaking treatments has been studied for distributions of 5 vol.-% Al₇Cr, Al₃Fe, and Al₃Ni in aluminium generated from chillcast alloy samples (cooling rate 10³ K s⁻¹) with and without subsequent cold working. Working prior to soaking enhanced the sluggish precipitation from extended solid solutions obtained for Al–Cr and enhanced both spheroidization and coarsening in Al–Fe and Al–Ni. The final size of intermetallic particles after 1000 h at 500°C increased in the order Al–Cr, Al–Fe, and Al–Ni for worked samples, largely reflecting the relative stabilities towards intermetallic coarsening in the three systems. The coarsening of Al₃Fe and Al₇Cr at 500°C was ∼2–3 orders of magnitude greater than predicted on the assumption of lattice diffusion control. Agreement for Al₃Fe was within a factor of 2, however, on the assumption of grain-boundary diffusion control, indicating that residence time on grain boundaries made the dominant contribution to coarsening. The coarsening of Al₃Fe and Al₃Ni at 600°C was, in contrast, within a factor of 2 of predictions based on lattice diffusion control. A larger grain size in Al–Ni than in Al–Fe is attributable to the larger intermetallic particle size at all stages in the former alloy.